1. Field of the Invention
The present invention relates to technology to advance quality of an image (a search picture) that image data recorded on the magnetic tape is reproduced at high speed, particularly to a data reproducing method, a reproducing speed control apparatus and a reproducing apparatus that it is possible to reproduce more image data by using plural reproducing heads.
2. Description of the Prior Art
In a video tape recorder for recording and reproducing a television signal by using a magnetic tape, a digital video tape recorder (a digital VTR) is proposed as it is possible to record and reproduce without degrading substantially the television signal. Further, in case that the magnetic tape used for the digital VTR is stored into a cassette, it is also called a digital video cassette tape recorder (a digital VCR).
The digital VCR records a voice signal and an image signal including a control signal and so on into the magnetic tape as digital data and obtains the image signal by reproducing the digital data recorded on the magnetic tape.
An area on the magnetic tape on which the digital data of the image signal is recorded by the digital VCR is called a track. The track is generally an inclined recording track (a helical track) recorded and formed by scanning the magnetic tape by a recording heads installed on a rotating drum inclined to the longitudinal direction, that is, to the traveling direction of the magnetic tape.
The digital VCR is possible to control degradation of the signal to the minimum by using data error correcting technology because the image signal recorded to and reproduced from the magnetic tape is the digital signal. Therefore, it has a characteristic, that it is comparatively easy to edit the magnetic tape on which the image signal is recorded and to reproduce it by changing speed, different from a conventional analog VCR.
In the digital VCR (simply VCR below), when the image data (a moving picture) recorded on the magnetic tape is reproduced (search) at high speed faster than the original reproducing speed which is the same speed as the recording speed, the reproducing head scans over plural tracks. That is, any image data recorded on one track is not scanned and any image data necessary for reproducing one screen is not reproduced.
As a result, a search picture becomes rough comparing with the image reproduced at the original reproducing speed, includes noise, therefore, quality of the picture becomes inferior.
As above mentioned, a main cause of a deterioration of the quality of the search picture is that the image data reproduced from the magnetic tape by scanning once by the reproducing heads decreases as the reproducing speed becomes faster. Therefore, by increasing the number of the reproducing heads provided on the rotating drum, the image data is also reproduced as much as possible in reproducing the search picture and it is tried to advance the quality of the search picture.
Relation between disposing of the reproducing heads on the rotating drum and reproducing speed will be described referring to FIG. 14.
FIG. 14A shows configuration of reproducing heads at least necessary for reproducing image data at the original speed and an example of the configuration of two heads that a set of the reproducing heads A1 and B1 which azimuth angles are different each other is disposed.
Here, the azimuth angle is the angle that inclines a gap of the recording head for recording and forming each track on the opposite side of the direction of forming the track each other in order to prevent signals of the adjacent tracks on the magnetic tape from interference. Usually this azimuth angle is approximately from several to 10 degrees and it is possible to record at high density without providing a guard band and so on by setting properly this value.
In the VCR of the configuration of two heads having + and - azimuth, it is known that the reproducing speed (the search speed) S in reproducing the search picture may be set to satisfy the following an expression (1). EQU S=(T/2).times.n.+-.0.5 [Times Speed] (1)
(n is an integer and T is the number of helical tracks number for recording image data of one screen.)
Because the reproducing head of + azimuth angle is rushed into a first track of T tracks forming one screen by a first scanning by the drum, into a second track by a second scanning, into a third track by a third scanning . . . , therefore, the search speed of the traced track is changed one after another by every scanning of the drum, and the image data of adjacent frames about time is continuously reproduced.
Further, it is particularly effective for the VCR using the shuffling pattern to dispose the image data of Macro Blocks adjacent on the screen together on the magnetic tape in order to renew an area as large as possible on the screen by scanning once to choose the search speed to the double speed given by the expression (1). Such shuffling pattern is used in case of the large Macro Block (for example, 16 pixels.times.16 scanning lines) that is a fundamental unit in case of forming the screen by using the image compressing technology as below mentioned.
FIG. 14B shows an example of the configuration of 4 heads that another set of reproducing heads A5 and B5 that the azimuth angles are different each other is added to the configuration of two heads as shown in FIG. 14A at the opposite positions on the rotating drum.
When the search speed S shown by the expression (1) is chosen in the VCR of 4 heads configuration, the data on the track of the same track number reproduced by plural reproducing heads having the same azimuth angle each other at the speed of some times (for example, n=2, 4, . . . ) each other overlaps each other, therefore, it is impossible to make use of the reproducing heads increased for increasing amount of the image data that it is possible to reproduce by scanning once. It is also similarly in the VCR of 8 heads configuration as shown in FIGS. 14C and 16 heads configuration as shown in FIG. 14D.
However, it was vague that it was effectively possible to use the plural reproducing heads to the maximum when any speed except the search speed S satisfying the expression (1) was set.
As above mentioned, as relation between the configuration of the reproducing heads and the reproducing speed was not clear in the VCR that the plural reproducing heads were installed on the rotating drum, even if the plural reproducing heads had been installed, they did not always work effectively.